>I don't understant this. All interface have an MTU, rigth ?
>I can't see what a IFF_ flag would give us. To enable/disable the "long frame"
>feature ?
>Anyway we have 2 things: the MTU we want to advertize to protocol layers,
>and the MTU the interface can really do. We need a way to advertize both and
>this, independantly from a IFF_ flag.
ifp->if_mtu: MTU for L3 protocols
ifp->if_hdrlen: header size for L2
ifp->if_mtu + ifp->if_hdrlen: L2 frame size
if we have single layer, it would be like this:
mtu hdrlen total
--- --- ---
1500 14 1514 normal ethernet frame
1496 18 1514 VLAN with normal ethernet frame
1500 18 1518 VLAN with larger frame
the problems are:
- we have two ways to do VLAN: decreasing MTU or using bigger L2 frame
- we would like to split vlan* and real interface handling clearly,
and it tend to add some confusion
- if_hdrlen needs to be more flexible?
itojun